1. Field of the Invention
The present invention relates to a collision avoidance system of a vehicle, and more particularly to a collision avoidance system of a vehicle that is capable of distinguishing an object ahead of the vehicle, whether it is a creature or not, by means of infrared rays and digital signal processing.
2. Description of the Prior Art
There is a continuing increase in the density of vehicles traveling the world's roadways, and simultaneously, an increasing emphasis on improving the safety of highway and ordinary roadway vehicles operations by preventing vehicles from colliding with stationary and moving objects(such as roadside obstacles and other vehicles). One means for accomplishing this is to detect or monitor the relative speed, the direction of travel, and the distance between vehicles sharing the roadway, and to use such information to provide direct indications to the vehicle's driver of potential danger. It is becoming increasingly more common for automotive engineers to consider the use of microwave radar systems as a means of monitoring or detecting such environmental conditions.
The microwave radar system is realized in a navigation apparatus which is mainly utilized in an airplane or ship. One collision avoidance system adapts the navigation apparatus to a vehicle. Vehicle borne radar systems which transmit and receive three frequencies on a time division basis, with two of the frequencies being used to determine the range, and the third being combined with one of the first two to determine the closing speed and the likelihood of collision. One such system is disclosed in U.S. Pat. No. 3,952,303, dated Apr. 20, 1976 to Watanabe et al., which is an analog radar signal processing system utilizing the Doppler effect.
However, the analog system as of the Watanabe is sensitive to temperature changes, and is difficult to calibrate. Furthermore, the Watanabe's system is dedicated to specific tasks and therefore, is difficult to upgrade and customize to meet varying requirements. Still furthermore, the transmit and receive frames in such a three frequency system can be wasteful, in that only small portions thereof are needed to determine the range and the relative rate of motion of a target, so the remaining portions of the frames are unused.
The analog processing technique is fast and allows for real time processing. However, the cost of an analog circuit is typically much greater than the cost of a digital circuit. Thus, if the collision analog signal can be converted and handled by a digital signal, the cost of the system will be lowered. Additionally, a digital signal processing circuit is much less sensitive to temperature, manufacturing variations, and interference from noise than an analog signal processing circuit.
A radar-operated collision avoidance system is disclosed in U.S. Pat. No. 4,072,945, dated Feb. 7, 1978 to Katsumata et al. Katsumata's system detects the relative speed and the relative distance of an object and determines whether the vehicle is approaching the object at a dangerously high speed. The minimum allowable distance between a vehicle and a object is stored in the memory of a computer as a digital code, and is then compared to the distance detected by the radar.
U.S. Pat. No. 4,626,850, dated Dec. 2, 1986 to Young H. Chey, discloses a dual operational mode vehicle detection and collision avoidance apparatus. Chey's apparatus uses a single active or passive ultrasonic ranging device, and is adapted to scan the rear and the lateral sides of the vehicle to warn the vehicle driver of any danger when the vehicle is changing lanes.
U.S. Pat. No. 5,302,965, issued to Jimmie R. Asbury et al. in Apr. 12, 1994, describes a multi-frequency and multi-object vehicle radar system. Jimmie's system utilizes two transmit and receive channels to determine the relative speed and the range of an object by determining the frequency and the difference in phase of the two channels.
All of the radars employed in the above-described U.S. patents use microwave radars as a part of its scanning and detecting apparatuses. The disadvantage of these patents is related to the beam width, that is the angular width of a main lobe of the radar, and to the associated angular resolution of the microwave radar.
The beam width is inversely proportional to the antenna diameter in wavelength. Therefore, the antenna should be large enough so as to reduce the beam width, but the size of the antenna shall be restricted when it is equipped to a vehicle. Consequently, it is very difficult to make a reasonable size microwave radar with a beam width less than 3 degrees.
For a desired scanning distance, a beam width of more than 3 degrees will scan an area which is much too big and thus, is too nonspecific and too difficult to differentiate the received echoes. Even though the echoes reflected from another preceding vehicle are received, the radar also receives signals from other objects close to the road, i.e., the echoes reflected from trees, posts, bridges across a nearby expressway. On an expressway with divided lanes, a microwave radar will receive echoes reflected from the vehicles that are within two or three lanes, and will not differentiate these echoes from echoes coming from objects in the same lane of the road. Because of the poor angular resolution of a microwave radar, the direction of objects can not be specifically determined and objects too close to one another cannot be separated.
A vehicle collision avoidance system using a laser radar in order to solve the aforementioned problems is disclosed in U.S. Pat. No. 5,314,037, issued to David C. H Shaw et al. in May 24, 1995. Shaw's system includes a laser radar with transmitters and receivers, a computer, a warning device, and an optional automatic braking device. A steering wheel rotation sensor, or laser gyroscope, is utilized to give information about a system-equipped vehicle's directional change. Shaw's system compares a predicted collision time with the minimum allowable time for determining the immanency of a collision to warn a driver of danger. Unless a driver responds to the warning, the optional automatic braking device is operated.
When the object is a human, a moving speed of the human is considerably slower than that of a vehicle. Also, the advancing direction of the human can be arbitrarily changed in accordance with his mental activity, which cannot be predicted. Thus, if the object is a human, it is necessary to decelerate the vehicle or to activate the braking operation to prepare for an emergency.
However, Shaw's system has the disadvantage of not being able to distinguish whether the detected object is a creature or not, and so not being able to selectively perform the deceleration or braking operation in accordance with the kind of the object detected, i.e., a living creature (human) or an inanimate object.